4840
Langmuir 1996, 12, 4840-4849
Organic Molecular Films under Shear Forces: Fluid and Solid Langmuir Monolayers Vladimir V. Tsukruk,* Valery N. Bliznyuk, John Hazel, and Dale Visser College of Engineering & Applied Sciences, Western Michigan University, Kalamazoo, Michigan 49008
Mark P. Everson Physics Department, Ford Research Laboratory, Dearborn, Michigan 48121 Received March 8, 1996X Scanning probe microscope observations of monolayers of a classic boundary lubricant, stearic acid (STA), reveal long-range dynamics of wear and reconstruction of monomolecular films under the shear forces caused by the sliding tip. The STA monolayer in a fluid state displays a flow of material from the worn area and its redistribution resulting in multilayer formation within the range of 80 µm. Surface diffusion of mobile organic material is responsible for the observed long-range effects of the local shear stresses produced within the contact area. Solid and fluid monolayers have very different velocity dependencies of the friction forces. For solid monolayers, we observe a monotonic increase of the friction forces with velocity rising from 0.02 to 1000 µm/s. In contrast, for the fluid STA monolayers the friction forces behave nonmonotonically with a maximum value around 0.2 µm/s. We observe significant compression of the STA monolayers under the tip reaching 35% of initial thickness before the fatal damaging. The observed compression can be related to the collective tilting of the molecules under normal loads due to a formation of gauche conformers in alkyl chains. Estimated Young’s modulus is in the range of 0.2-0.7 GPa for very small deformations (400 nN) eventually leads to disruption of the monolayer surface and the formation of a hole about 2.0 nm deep (see an example for the STCd20 monolayer in Figure 5). This value is slightly lower than that of the thickness of the monolayer measured independently on the undamaged areas (within the holes). Interestingly, the friction forces within the worn area are much smaller than those for a bare silicon surface, which indicates the presence of traces of organic molecules at the bottom of the hole. The molecules lying down on the silicon surface will produce the observed deficit in the depth and will serve as a lubricant layer to reduce local friction. For the fluid monolayers, at the intermediate stages of wear, the formation of many submicrometer pinholes is observed at the initially uniform surface. After several succeeding scans, the material is removed completely from the scanning area which becomes a rectangular hole of 2.0 ( 0.4 nm depth (Figure 6). Zooming out of the worn area (Figure 6a) reveals no accumulated material along the edges as usually observed for plastically deformed organic materials.13,16 In contrast, a number of large round holes occur around the worn area. The diameter of these holes is in the range of 0.4-1 µm. A striking feature of this new pattern is the formation of thicker, multilayer areas in the vicinity of the worn area. Instead of the original monolayer of 2.2 nm thickness, we observed bilayers, trilayers, and even quadlayers around the worn area. The formation of local multilayers must result in lower surface energy due to reducing tail-to-surface and head-to-surface interactions. In our case, sufficient mobility of the STA molecules within the fluid monolayers combined with the forced material diffusion and external energy supply provided by the shear stresses induces a complex reconstruction of the monolayer even in air. Surface diffusion of fluid material under the shear stresses can be a possible mechanism for the formation of a longrange reconstruction pattern of the fluid monolayer (see discussion in ref 11). To reveal the monolayer behavior under different normal loads, we monitor the thickness of the monolayer and the friction forces at normal loads from 20 to 900 nN (Figure 7). Initial thickness, d, of 1.9 nm observed for the STCd12 monolayer at low loads decreases gradually to 1.2 nm with an increase of the normal load to 300-400 nm. At higher loads in the range of 400-600 nN we observe plastic deformation of the monolayer. Finally, at normal loads higher than 600 nN the monolayer is completely destroyed and its thickness drops sharply to 0.5 nm. The friction forces rise modestly for low normal loads but increase very sharply (4 times) within a narrow interval of normal loads corresponding to damage and sharp decrease of monolayer thickness. Similar behavior is observed for the STA20 monolayer with plastic deformation observed at lower loads (150-200 nm). This behavior is very different if compared to chemically tethered alkylsilane (C16) self-assembled monolayers which are stable under similar experimental conditions up the highest normal loads. The observed variations of the surface morphology can be related to the compression behavior and wear of the monolayers as follows. At low normal loads (
